At present, due to industrial development and human activities, the atmosphere is severely polluted, thus threatening health of human beings. Furthermore, many serious epidemic diseases such as SARS and bird flu in recent years spread through air. The infected outdoor air can spread indoors along with airflow. In the hospital, air of the wards for patients of epidemic diseases is directly discharged outdoors and the viruses carried by the air threaten the safety of other people. Common air cleaning systems on the market can only clean suspending dust or particles in the air, but cannot clean the potential viruses in the air. Moreover, most air cleaning systems only clean or filter the indoor air, do not circulate the air and do not clean the air to be discharged; therefore they cannot prevent disease spreading.